8-16 D60 Line Distance Protection System GE Multilin
8.3 GROUND DIRECTIONAL OVERCURRENT 8 THEORY OF OPERATION
8
8.3GROUND DIRECTIONAL OVERCURRENT 8.3.1 DESCRIPTION
Consider the negative-sequence directional overcurrent element. As illustrated below, the negative-sequence voltage could
be low during internal fault conditions.
Figure 8–6: OFFSET IMPEDANCE AUGMENTATION
In order to ensure operation of the element under such circumstances the angle comparator uses a polarizing voltage aug-
mented by the negative-sequence current as per following equations:
Forward-looking element:
(EQ 8.5)
Reverse-looking element:
(EQ 8.6)
Where ECA = forward ECA angle (maximum torque angle) and Z_offset = offset impedance. The effect of the augmentation
for forward and reverse fault is shown in the figures above. As long as the offset impedance is not higher than the negative-
sequence line impedance the element will ensure correct and fast fault direction identification for both forward and reverse
faults. The same principle applies to the neutral directional overcurrent element.
8.3.2 EXAMPLE
Consider relay input signals as in the Distance Elements Analysis section and assume an offset impedance of 4 and
ECA and limit angles of 88° and 90°, respectively. The relay calculates the following negative-sequence quantities:
V_2 = 6.39 V –159.6°; I_2 = 1.37 A –68.1°; I_1 = 2.94 A –144.2°
and the following signals for the directional unit of the negative-sequence directional overcurrent element:
• Forward-looking element:
• Reverse-looking element:
After comparing the angles, a solid forward indication is given.
Assume further the pickup setting of 0.25 A for both forward and reverse directions, and the “Negative-sequence” mode
setting entered for the overcurrent unit of the element. The relay calculates the operating signal using the positive-
sequence restraint:
UR
V_2
(a)
UR
V_2 >
I_2 * Z_2line
(c)
I_2
ECA
I_2 x Z
V_2 -V_2
S_pol
S_op
(b)
I_2
ECA
I_2 x Z
V_2
-V_2
S_pol
S_op
(d)
-V_2
S_pol V_2– I_2+ Z_offset 1ECA=
S_op I_2 1 ECA=
S_pol V_2– I_2+ Z_offset 1ECA=
S_op I_2 1 ECA–=
S_pol 11.87 V 20.2=
S_op 1.37 V 20.2=
S_pol 11.87 V 20.2=
S_op 1.37 V 160.0– =
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